Primary charge separation in closed Photosystem II with a lifetime of 11 ns

High-Speed Excitation-Spectral Microscopy Uncovers In Situ Rearrangement of Light-Harvesting Apparatus in Chlamydomonas during State Transitions at Submicron Precision

Photosynthetic organisms adjust to fluctuating natural light under physiological ambient conditions through flexible light-harvesting ability of light-harvesting complex II (LHCII). A process called state transition is an efficient regulation mechanism to balance the excitations between photosystem II (PSII) and photosystem I (PSI) by shuttling mobile LHCII between them. However, in situ observation of the migration of LHCII in vivo remains limited. In this study, we investigated the in vivo reversible changes in the intracellular distribution of the chlorophyll (Chl) fluorescence during the light-induced state transitions in Chlamydomonas reinhardtii. The newly developed noninvasive excitation-spectral microscope provided powerful spectral information about excitation-energy transfer between Chl-a and Chl-b. The excitation spectra were detected through the fluorescence emission in the 700-750-nm spectral range, where PSII makes the main contribution, though PSI still makes a non-negligible contribution at room temperature. The technique is sensitive to the Chl-b spectral component specifically bound to LHCII. Using a PSI-specific 685-nm component also provided visualization of the local relative concentration of PSI within a chloroplast at room temperature. The decrease in the relative intensity of the Chl-b band in state 2 was more conspicuous in the PSII-rich region than in the PSI-rich region, reflecting the dissociation of LHCII from PSII. We observed intracellular redistributions of the Chl-b-related light-harvesting abilities within a chloroplast during the state transitions. This observation implies the association of the state transitions with the morphological changes in the thylakoid membrane.

Redox transients of P680 associated with the incremental chlorophyll-a fluorescence yield rises elicited by a series of saturating flashes in diuron-treated photosystem II core complex of Thermosynechococcus vulcanus

Recent chlorophyll-a fluorescence yield measurements, using single-turnover saturating flashes (STSFs), have revealed the involvement of a rate-limiting step in the reactions following the charge separation induced by the first flash. As also shown here, in diuron-inhibited PSII core complexes isolated from Thermosynechococcus vulcanus the fluorescence maximum could only be reached by a train of STSFs. In order to elucidate the origin of the fluorescence yield increments in STSF series, we performed transient absorption measurements at 819 nm, reflecting the photooxidation and re-reduction kinetics of the primary electron donor P680. Upon single flash excitation of the dark-adapted sample, the decay kinetics could be described with lifetimes of 17 ns (∼50%) and 167 ns (∼30%), and a longer-lived component (∼20%). This kinetics are attributed to re-reduction of P680^•+ by the donor side of PSII. In contrast, upon second-flash (with Δt between 5 μs and 100 ms) or repetitive excitation, the 819 nm absorption changes decayed with lifetimes of about 2 ns (∼60%) and 10 ns (∼30%), attributed to recombination of the primary radical pair P680^•+ Pheo^•- , and a small longer-lived component (∼10%). These data confirm that only the first STSF is capable of generating stable charge separation - leading to the reduction of Q_A ; and thus, the fluorescence yield increments elicited by the consecutive flashes must have a different physical origin. Our double-flash experiments indicate that the rate-limiting steps, detected by chlorophyll-a fluorescence, are not correlated with the turnover of P680.

Introduction to optical methods in photosynthesis

Optical spectroscopy is widely used to study structure and function of photosynthetic systems. Due to the large variety of different methods, these studies have contributed a lot to the identification of the cofactors involved in the primary reactions of photosynthesis and to the elucidation of the kinetics of the light-induced energy and electron transfer reactions. Within other aspects of photosynthesis research as e.g. photoinhibition, these techniques play an important role as well. In this brief introduction, I will focus on the basic principles of the different methods and the information obtained by applying these various techniques. In the reviews that follow, under the section "Optical Methods", these methods are discussed in detail.

Use of ruthenium dyes for subnanosecond detector fidelity testing in real time transient absorption

Transient absorption spectroscopy is a powerful tool for the study of photoreactions on time scales from femtoseconds to seconds. Typically, reactions slower than approximately 1 ns are recorded by the "classical" technique; the reaction is triggered by an excitation flash, and absorption changes accompanying the reaction are recorded in real time using a continuous monitoring light beam and a detection system with sufficiently fast response. The pico- and femtosecond region can be accessed by the more recent "pump-probe" technique, which circumvents the difficulties of real time detection on a subnanosecond time scale. This is paid for by accumulation of an excessively large number of shots to sample the reaction kinetics. Hence, it is of interest to extend the classical real time technique as far as possible to the subnanosecond range. In order to identify and minimize detection artifacts common on a subnanosecond scale, like overshoot, ringing, and signal reflections, rigorous testing is required of how the detection system responds to fast changes of the monitoring light intensity. Here, we introduce a novel method to create standard signals for detector fidelity testing on a time scale from a few picoseconds to tens of nanoseconds. The signals result from polarized measurements of absorption changes upon excitation of ruthenium complexes {[Ru(bpy)(3)](2+) and a less symmetric derivative} by a short laser flash. Two types of signals can be created depending on the polarization of the monitoring light with respect to that of the excitation flash: a fast steplike bleaching at magic angle and a monoexponentially decaying bleaching for parallel polarizations. The lifetime of the decay can be easily varied via temperature and viscosity of the solvent. The method is applied to test the performance of a newly developed real time transient absorption setup with 300 ps time resolution and high sensitivity.

Charge separation, stabilization, and protein relaxation in photosystem II core particles with closed reaction center

The fluorescence kinetics of cyanobacterial photosystem II (PSII) core particles with closed reaction centers (RCs) were studied with picosecond resolution. The data are modeled in terms of electron transfer (ET) and associated protein conformational relaxation processes, resolving four different radical pair (RP) states. The target analyses reveal the importance of protein relaxation steps in the ET chain for the functioning of PSII. We also tested previously published data on cyanobacterial PSII with open RCs using models that involved protein relaxation steps as suggested by our data on closed RCs. The rationale for this reanalysis is that at least one short-lived component could not be described in the previous simpler models. This new analysis supports the involvement of a protein relaxation step for open RCs as well. In this model the rate of ET from reduced pheophytin to the primary quinone Q(A) is determined to be 4.1 ns(-1). The rate of initial charge separation is slowed down substantially from approximately 170 ns(-1) in PSII with open RCs to 56 ns(-1) upon reduction of Q(A). However, the free-energy drop of the first RP is not changed substantially between the two RC redox states. The currently assumed mechanistic model, assuming the same early RP intermediates in both states of RC, is inconsistent with the presented energetics of the RPs. Additionally, a comparison between PSII with closed RCs in isolated cores and in intact cells reveals slightly different relaxation kinetics, with a approximately 3.7 ns component present only in isolated cores.

A photoprotection mechanism involving the D(2) branch in photosystem II cores with closed reaction centers

Nanosecond transient absorption spectroscopy has been used to study reaction centre (RC) chlorophyll triplet quenching by carotenoid in intact photosystem II cores from T. elongatus with closed RCs. We found a triplet beta-carotene ((3)Car) signal (absorption difference maximum at 530 nm) that is sensitized by the RC chlorophyll (Chl) triplet with a formation time of ca. 190 ns, has a decay time of 7 micros and is formed with a quantum yield between 10 and 20%. The (3)Car signal is assigned to the beta-carotene on the D(2) branch of the RC. We thus propose a new photoprotection mechanism operative in closed RCs where-as a consequence of the negative charge on the quinone Q(A)-the triplet chlorophyll ((3)Chl) is formed by the radical pair (RP) mechanism on the normally inactive D(2) branch where it can be subsequently quenched by the D(2) beta-carotene. We suggest that the D(2) branch becomes active when the RCs are closed under high light fluence conditions. Under these conditions the D(2) branch plays a photoprotective role. This interpretation allows combining many seemingly inconsistent observations in the literature and reveals the so far missing RC triplet quenching mechanism in photosystem II. The newly proposed mechanism also explains the reason why this RC triplet quenching is not observed in isolated D(1)-D(2)-cyt b(559) RCs. If Q(A) is either not present at all (as in the isolated RC) or is not charged (as in open RCs or with doubly reduced Q(A)) then the RC (3)Chl is formed on the D(1) branch. The D(1) branch (3)Chl can not be quenched due to the large distance to the beta-carotene. This interpretation is actually in line with the well-known (3)RC quenching mechanism in bacterial RCs, where also the carotenoid in the (analogous to the D(2) branch) B-branch of the RC becomes the quencher.

Modulation of primary radical pair kinetics and energetics in photosystem II by the redox state of the quinone electron acceptor Q(A)

Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q(A) either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680(+)Pheo(-). The maximum transient concentration of the primary radical pair is about five times larger and its decay is about seven times slower with doubly reduced compared with singly reduced Q(A). The possible biological significance of these differences is discussed. On the basis of a simple reversible reaction scheme, the measured apparent rate constants and relative amplitudes allow determination of sets of molecular rate constants and energetic parameters for primary reactions in the reaction centers with doubly reduced Q(A) as well as with oxidized or singly reduced Q(A). The standard free energy difference DeltaG degrees between the charge-separated state P680(+)Pheo(-) and the equilibrated excited state (Chl(N)P680)* was found to be similar when Q(A) was oxidized or doubly reduced before the flash (approximately -50 meV). In contrast, single reduction of Q(A) led to a large change in DeltaG degrees (approximately +40 meV), demonstrating the importance of electrostatic interaction between the charge on Q(A) and the primary radical pair, and providing direct evidence that the doubly reduced Q(A) is an electrically neutral species, i.e., is doubly protonated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in DeltaG degrees than the rate of primary charge separation.

Two fundamentally different types of variable chlorophyll fluorescence in vivo

Upon onset of saturating continuous light only the first part of the observed polyphasic fluorescence rise follows Q(A) reduction (photochemical phase), whereas the remaining part (thermal phases) is kinetically limited by relatively slow reactions with light saturated half-times in the order of 10-50 ms. A simple hypothesis is presented for the interpretation of these fundamentally different types of variable fluorescence. The hypothesis, which is based on the reversible radical pair model of PSII, assumes stimulation of both prompt and recombination fluorescence upon Q(A) reduction, with only recombination fluorescence being in competition with nonradiative energy loss processes at the reaction centers. It is proposed that changes in the rate constants of these processes modulate the yield of recombination fluorescence in closed centers, thus causing large variations in the maximal fluorescence yield and also giving rise to the 'thermal phases'. This hypothesis can reconcile numerous experimental findings which so far have seemed difficult to interpret.

Variable thermal dissipation in a Photosystem I submembrane fraction

Photoacoustic spectroscopy was used to study the thermal deactivation processes in a Photosystem I submembrane fraction isolated from spinach. A large part of the thermal dissipation was variable. The yield of this variable thermal emission depended on the redox state of the Photosystem. It increased with the measuring modulated light intensity coinciding with the gradual closure of the reaction centers. Thermal deactivation was maximal when the reaction centers were closed by a saturating illumination. Extrapolation of the data at zero light intensity indicated that the yield of non-variable thermal emission represented about 37% of the maximal emission. The presence of methylviologen as artificial electron acceptor decreased the yield of variable thermal emission whereas inhibition following heat stress treatments increased it. The significance of the variable and non-variable components of thermal dissipation is discussed and the measured energy storage is suggested to originate from the reduction of the plastoquinone pool during cyclic electron transport around Photosystem I.

Events near the reaction center in O2 evolving PS II enriched thylakoid membranes: The presence of an electric field during the S2 state in a population of centers

Flash-induced absorption changes at 515 nm observed as a function of flash number are examined in relation to the flash-induced fluorescence yields in inside-out thylakoids. After partial dissipation of the delocalized transmembrane electric field by adding gramicidin, the analysis of period 4 oscillations and of the kinetics in the 10 ms-1 s range suggest that the variation of the absorption changes at 515 nm as a function of flash number is the result of at least two processes:1) an electric field increase related to the S2 state and 2) the fact that the field generated by the water protons inside the membrane decreases when these protons are released outside the membrane. The former field correlates with the flash-induced fluorescence yield increase induced by the donor side of Photosystem II. Both measurements show similar oscillations as a function of flash number, with maxima on the 1st, 5th and 9th flash. These oscillations, after a shift of two flashes, appear to be different from those of the O2 yield observed under similar conditions. It is proposed that, in a population of centers the electric field during the S2 state reflects the presence of a stabilized positive equivalent in the protein close to the Mn complex.

Rate of oxidation of P680 in isolated photosystem 2 reaction centers monitored by loss of chlorophyll stimulated emission

We have continued our studies of the primary photochemistry of isolated photosystem 2 reaction centers using femtosecond transient absorption spectroscopy. Experiments were performed over a wide range of excitation and probe wavelengths, using several data collection time scales. This has enabled us to resolve five different lifetimes ranging between 100 fs and 200 ps plus a nanosecond component. We demonstrate here and elsewhere [e.g., Durrant, J.R., Hastings, G., Joseph, D. M., Barber, J., Porter, G., & Klug, D. R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11632-11636] that the kinetic spectra associated with all but two of these lifetimes are clearly distinguishable. We have previously reported that a 21-ps lifetime is associated with pheophytin reduction [Hastings, G., Durrant, J. R., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Biochemistry 31, 7638-7647]. In this paper, we show that it is possible to spectrally and temporally resolve stimulated emission from PS2 reaction centers with great accuracy and that this stimulated emission is largely unaffected by those kinetic components which are faster than 21 ps. The observation of a distinct stimulated emission band allows us to distinguish charge-separated states from chlorin singlet states. In this way, we are able to show that the proportion of charge-separated states prior to the 21-ps component is between 0% and 25%. We also show that the shape of the spectrum which we obtain for the state P680+Ph- is essentially invariant between 100 ps and 9 ns, and is the same as that previously reported for P680+Ph- by other researchers.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-lived primary radical pair state detected by time-resolved fluorescence and absorption spectroscopy in an isolated Photosystem two core

A Photosystem two (PS II) core preparation containing the chlorophyll a binding proteins CP 47, CP 43, D1 and D2, and the non-chlorophyll binding cytochrome-b559 and 33 kDA polypeptides, has been isolated from PS II-enriched membranes of peas using the non-ionic detergent heptylthioglucopyranoside and elevated ionic strengths. The primary radical pair state, P680(+)Pheo(-), was studied by time-resolved absorption and fluorescence spectroscopy, under conditions where quinone reduction and water-splitting activities were inhibited. Charge recombination of the primary radical pair in PS II cores was found to have lifetimes of 17.5 ns measured by fluorescence and 21 ns measured by transient decay kinetics under anaerobic conditions. Transient absorption spectroscopy demonstrated that the activity of the particles, based on primary radical pair formation, was in excess of 70% (depending on the choice of kinetic model), while time-resolved fluorescence spectroscopy indicated that the particles were 91% active. These estimates of activity were further supported by steady-state measurements which quantified the amount of photoreducible pheophytin. It is concluded that the PS II core preparation we have isolated is ideal for studying primary radical pair formation and recombination as demonstrated by the correlation of our absorption and fluorescence transient data, which is the first of its kind to be reported in the literature for isolated PS II core complexes from higher plants.

Observation of multiple radical pair states in photosystem 2 reaction centers

Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.

Photoelectric study on the kinetics of trapping and charge stabilization in oriented PS II membranes

Excitation energy trapping and charge separation in Photosystem II were studied by kinetic analysis of the fast photovoltage detected in membrane fragments from peas with picosecond excitation. With the primary quinone acceptor oxidized the photovoltage displayed a biphasic rise with apparent time constants of 100-300 ps and 550±50 ps. The first phase was dependent on the excitation energy whereas the second phase was not. We attribute these two phases to trapping (formation of P-680(+) Phe(-)) and charge stabilization (formation of P-680(+) QA (-)), respectively. A reversibility of the trapping process was demonstrated by the effect of the fluorescence quencher DNB and of artificial quinone acceptors on the apparent rate constants and amplitudes. With the primary quinone acceptor reduced a transient photoelectric signal was observed and attributed to the formation and decay of the primary radical pair. The maximum concentration of the radical pair formed with reduced QA was about 30% of that measured with oxidized QA. The recombination time was 0.8-1.2 ns.The competition between trapping and annihilation was estimated by comparison of the photovoltage induced by short (30 ps) and long (12 ns) flashes. These data and the energy dependence of the kinetics were analyzed by a reversible reaction scheme which takes into account singlet-singlet annihilation and progressive closure of reaction centers by bimolecular interaction between excitons and the trap. To put on firmer grounds the evaluation of the molecular rate constants and the relative electrogenicity of the primary reactions in PS II, fluorescence decay data of our preparation were also included in the analysis. Evidence is given that the rates of radical pair formation and charge stabilization are influenced by the membrane potential. The implications of the results for the quantum yield are discussed.

Applications of ultrafast laser spectroscopy for the study of biological systems

The discovery of mode-locked laser operation now nearly two decades ago has started a development which enables researchers to probe the dynamics of ultrafast physical and chemical processes at the molecular level on shorter and shorter time scales. Naturally the first applications were in the fields of photophysics and photochemistry where it was then possible for the first time to probe electronic and vibrational relaxation processes on a sub-nanosecond timescale. The development went from lasers producing pulses of many picoseconds to the shortest pulses which are at present just a few femtoseconds long. Soon after their discovery ultrashort pulses were applied also to biological systems which has revealed a wealth of information contributing to our understanding of a broadrange of biological processes on the molecular level.It is the aim of this review to discuss the recent advances and point out some future trends in the study of ultrafast processes in biological systems using laser techniques. The emphasis will be mainly on new results obtained during the last 5 or 6 years. The term ultrafast means that I shall restrict myself to sub-nanosecond processes with a few exceptions.

The quenching characteristics of potassium iridic chloride and their meaning for the origin of chlorophyll fluorescence components

To understand the origins of the different lifetime components of photosystem 2 (PS2) chlorophyll (Chl) fluorescence we have studied their susceptibility to potassium iridic chloride (K2IrCl6) which has been shown to bleach antenna pigments of photosynthetic bacteria (Loach et al. 1963). The addition of K2IrCl6 to PS2 particles gives rise to a preferential quenching of the variable Chl fluorescence (Fv). At concentrations lower than 20 μM, this is brought about mainly by a decrease in the yield, but not in the lifetime, of the slowest component when all the PS2 reaction centres are closed (FM). The yield of the middle and fast decays are not significantly altered. This type of quenching is not seen with DNB. The iridate-induced quenching of the initial fluorescence level (F0) is due to a proportional decrease in the yield and lifetime of the three components and correlates with the observed modification in the relative quantum yield of oxygen evolution. In this concentration range a bleaching of Chl a is seen. At higher iridate levels, greater than 20 μM, a proportional decrease in the lifetimes and yields of the three kinetic components is seen at FM. These changes are associated with a carotenoid bleaching. In isolated light harvesting Chl a/b complexes of PS2 (LHC2), iridate addition converts a 4 ns decay into a 200 ps emission and both types of bleaching are observed. By also measuring the rate of PS2 trap closure versus iridate concentration, we have discussed the results in terms of excitation energy transfer.